The present invention relates generally to semiconductor integrated circuit (IC) memory chips or other monolithic devices, and more particularly to serial erasable programmable memory devices such as electrically erasable (alterable) programmable read only memories (serial EEPROMs). EEPROM devices generally have the advantage that data may be written and rapidly erased electrically many times over, to permit a user to modify stored data at will while the device is in circuit.
ROM (Read Only Memory) devices are permanently mask-programmed at the factory so that no subsequent data modification is permitted. Originally, programmable ROMs (PROMs) employed fusible links in which data is stored according to the condition of the fuses at intersections of the memory array, i.e., the condition of whether or not a fuse is blown. Such PROM devices are programmable only once (that is, are non-erasable) since a blown metal or polysilicon link fuse is not reparable.
Erasable PROM (EPROM) devices require exposure of the unhoused structure to ultraviolet (UV) light to change the electrical characteristics of a charged element in order to obtain erasure. Typically, the EPROM is housed in a windowed package (e.g., a ceramic package having a quartz window to expose the silicon), although a more recent version dubbed as "one time programmable" (OTP) is packaged in plastic without a window. As the name indicates, the windowless variety can be programmed only once. The usual windowed EPROM is reprogrammable after removal from its in-system circuit, by exposing the device to UV light for a sufficient period of time (several hours) to assure complete erasure, then electrically reprogramming and reinstalling it in the circuit.
Since EEPROM devices are selectively erasable in circuit, they are the memory devices of choice in applications where in-system changes of stored data and/or relatively fast data changes may be required. Serial EEPROMs are nonvolatile memories that provide small footprint and board space as required in cellular phone applications; byte level erase, write and read of data as needed in television tuner applications; low voltage and current for hand held battery applications such as vehicle keyless entry transmitters; and multiple nonvolatile functions such as remote controls for video cassette recorders CVCR. Although the following list is not exhaustive, other applications of serial EEPROM devices include, in the consumer market, compact-disc (CD) players, cameras, radios and remote controls; in the automotive market, airbags, anti-lock brakes, odometers and radios; in the office automation market, printers, copiers and personal computers; in the telecommunications market, cordless and full feature phones, fax machines, modems, pagers and satellite receivers; and in the industrial market, bar code readers, Point of sale (POS) terminals, smart cards, lock boxes, garage door openers and test measurement equipment.
In the EEPROM structure, a pair of polysilicon gates are separated by a silicon dioxide layer. The oxide also extends below the lower gate to separate it from the underlying p-type silicon substrate in which a channel may be established between implanted heavily doped n-type source and drain regions. The oxide thickness between the lower gate and the silicon typically ranges up to about 100 angstroms, which is considerably less than the gate oxide thickness used for EPROM structures.
In operation of an EEPROM, a voltage of suitable magnitude applied across the very thin gate oxide layer induces tunneling of electrons from the substrate to the lower gate. A logical 1 is stored (written) when a write voltage is applied to the upper gate, thus inducing a charge on the lower gate that prevents a channel from forming during a read operation. A reversal of the write voltage causes erasure.
Despite their many advantages over other programmable semiconductor memory devices for certain applications, EEPROMs require programming or erase/write (E/W) times of sufficient length to constitute a serious drawback to their use. During programming, the associated microprocessor or microcontroller, for which the EEPROM is employed to provide additional memory in embedded control system applications, remains tied up and unable to perform other tasks until the programming is completed.
It is a principal object of the present invention to free up the microprocessor or microcontroller associated with an EEPROM device so that other tasks may be performed while the EEPROM is being written.